Sandstone acidizing is a stimulation treatment which involves the injection of an acid into the formation at matrix pressures (i.e., below that which fractures the formation) to react with formation materials such as sand, clays, drilling fluid, cement filtrate, and the like. The treatment normally involves the sequential injection of three fluids:
(1) Preflush: An aqueous acid solution, usually HCl, is injected to displace connate water from the near wellbore region and to react with calcite or other calcareous materials in the formation.
(2) Mud acid: Immediately following the preflush, an acid capable of reacting with siliceous constituents in the formation is injected. The acid generally is a mixture of HF and HCl, typically an aqueous solution of 3% HF and 12% HCl. The HF reacts with clays, sand, drilling mud and cement filtrate; whereas the main function of the HCl is to keep the pH low.
(3) Afterflush: The mud acid treatment is followed by an afterflush to displace the mud acid into the formation and restore water wetability to the formation and acid reaction products.
The effectiveness of sandstone acidizing is dependent upon the ability of the mud acid to improve formation permeability by reacting with formation materials. Over the years it has been observed that gas wells and oil wells respond differently to sandstone acid treatments. Studies have shown that gas well stimulation by sandstone acidizing is generally proportional to the volume of mud acid employed; whereas oil well stimulation responds proportionately only to relatively low acid volumes. Medium to high volume treatments of mud acid frequently do not provide additional benefits and in fact sometimes reduce stimulation.
Some studies indicate that the decline in acid stimulation in oil wells may be caused by disintegration of the sandstone matrix due to acid reaction. Although this mechanism may be operative, it does not explain the difference between gas well and oil well response to sandstone acid treatments. It appears that the disintegration of the matrix would occur in both types of sandstone formations.
As mentioned above and as described in more detail below, the method of the present invention employs a solvent in advance of the mud acid in the treatment to condition the formation and improve its response to the mud acid treatment.
Prior art which discloses the use of solvents in well acidizing operations include U.S. Pat. Nos.: 3,254,718, 3,481,404, 3,548,945, 3,902,557, 3,970,148, and 4,823,874.
The method disclosed in U.S. Pat. No. 3,254,718 involves the use of solvents in a preflush of an acid treatment for calcareous formations. As is well known in the industry, calcareous formations and sandstone formations have different properties which require different acid treatments. For example, calcareous materials have a positive surface charge at a pH below 8 which makes them oil wet; whereas sands have a negative surface charge making them water wet. Moreover entirely different acids are used in each, with HCl predominating in the acid treatment of calcareous formations and mud acid in the treatment of sandstone formations. The different acid reactions produce totally different reaction products, which require different treatments to avoid adverse effects to the acid reaction products.
U.S. Pat. Nos. 3,481,404 and 3,548,945 disclose the use of solvents in sandstone acid solutions, or in the afterflush of such acid solutions.
U.S. Pat. No. 3,902,557 discloses the use of solvents in the acid solution or afterflush in sandstone acidizing to prevent the occurrence of emulsion blocks.
U.S. Pat. No. 3,970,148 discloses an acidizing method involving the sequential injection of an aromatic solvent preflush and an alcohol-acid solution.
U.S. Pat. No. 4,823,874 discloses the use of certain antisludging agents in the acid solution of well acidizing compositions.